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Isocurvature bounds on axions revisited

Maria Beltran, Juan Garcia-Bellido, Julien Lesgourgues

TL;DR

The paper revisits axion dark matter scenarios produced by the misalignment mechanism in light of current CMB and LSS constraints, focusing on isocurvature perturbations induced by inflation. It derives how the axion isocurvature amplitude depends on the inflationary scale $M_{\rm inf}$, the Hubble parameter during inflation $H_{\rm inf}$, and the PQ scale $f_a$, showing that for high inflation scales the isocurvature bounds strongly constrain the allowed axion parameter space unless reheating is highly efficient or PQ symmetry is restored in the right way. The analysis identifies two viable regions in the $(M_{\rm inf}, f_a)$ plane and discusses the special case of chaotic inflation with a quadratic potential, which remains compatible with axions only for $f_a$ in the range $\sim 10^{10}$–$10^{11}$ GeV under plausible assumptions. Overall, the work emphasizes that isocurvature bounds are a crucial consideration for axion dark matter models with inflationary scales above $10^{12}$ GeV, while lower-scale inflation relaxes the constraints and leaves room for conventional axion windows.

Abstract

The axion is one of the best motivated candidates for particle dark matter. We study and update the constraints imposed by the recent CMB and LSS experiments on the mass of axions produced by the misalignment mechanism, as a function of both the inflationary scale and the reheating temperature. Under some particular although not unconventional assumptions, the axionic field induces too large isocurvature perturbations. Specifically, for inflation taking place at intermediate energy scales, we derive some restrictive limits which can only be evaded by assuming an efficient reheating mechanism, with T>10^{11} GeV. Chaotic inflation with a quadratic potential is still compatible with the axion scenario, provided that the Peccei-Quinn scale f_a is close to 10^{10} or 10^{11} GeV. Isocurvature bounds eliminate the possibility of a larger f_a and a small misalignment angle. We find that isocurvature constraints on the axion scenario must be taken into account whenever the scale of inflation is above 10^{12} GeV; below this scale, axionic isocurvature modes are too small to be probed by current observations.

Isocurvature bounds on axions revisited

TL;DR

The paper revisits axion dark matter scenarios produced by the misalignment mechanism in light of current CMB and LSS constraints, focusing on isocurvature perturbations induced by inflation. It derives how the axion isocurvature amplitude depends on the inflationary scale , the Hubble parameter during inflation , and the PQ scale , showing that for high inflation scales the isocurvature bounds strongly constrain the allowed axion parameter space unless reheating is highly efficient or PQ symmetry is restored in the right way. The analysis identifies two viable regions in the plane and discusses the special case of chaotic inflation with a quadratic potential, which remains compatible with axions only for in the range – GeV under plausible assumptions. Overall, the work emphasizes that isocurvature bounds are a crucial consideration for axion dark matter models with inflationary scales above GeV, while lower-scale inflation relaxes the constraints and leaves room for conventional axion windows.

Abstract

The axion is one of the best motivated candidates for particle dark matter. We study and update the constraints imposed by the recent CMB and LSS experiments on the mass of axions produced by the misalignment mechanism, as a function of both the inflationary scale and the reheating temperature. Under some particular although not unconventional assumptions, the axionic field induces too large isocurvature perturbations. Specifically, for inflation taking place at intermediate energy scales, we derive some restrictive limits which can only be evaded by assuming an efficient reheating mechanism, with T>10^{11} GeV. Chaotic inflation with a quadratic potential is still compatible with the axion scenario, provided that the Peccei-Quinn scale f_a is close to 10^{10} or 10^{11} GeV. Isocurvature bounds eliminate the possibility of a larger f_a and a small misalignment angle. We find that isocurvature constraints on the axion scenario must be taken into account whenever the scale of inflation is above 10^{12} GeV; below this scale, axionic isocurvature modes are too small to be probed by current observations.

Paper Structure

This paper contains 15 sections, 47 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Production mechanisms
  3. Thermal production
  4. Production via cosmic strings
  5. Generation via misalignment angle
  6. Isocurvature perturbations from axion fluctuations
  7. The axionic window
  8. The scale of inflation
  9. Supernova 1987A bounds
  10. Axionic cosmic strings production
  11. Cold Dark Matter produced by misalignment
  12. Isocurvature modes
  13. Discussion
  14. General case
  15. Bounds for chaotic inflation with a quadratic potential

Figures (2)

  • Figure 1: The one dimensional distributions of the sampled parameters.
  • Figure :